Capacitor discharge circuit

ABSTRACT

A capacitor discharge circuit according to one or more embodiments includes a capacitor connected in parallel to an alternating-current power source; a rectification element; a first discharge circuit that includes a first diode and a second diode, and causes the capacitor to discharge; a first series circuit including a first capacitor and a second capacitor connected in series between one end of the alternating-current power source and a ground terminal of the rectification element; a second discharge circuit that causes the second capacitor to discharge such that an absolute value of voltage across opposite ends of the second capacitor does not reach a predetermined voltage; and a predetermined period generator that actuates the first discharge circuit after an elapse of a predetermined period of time from stoppage of a discharge operation of the second discharge circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority based on 35 USC 119 from prior JapanesePatent Application No. 2016-103364 filed on May 24, 2016, entitled“CAPACITOR DISCHARGE CIRCUIT”, the entire contents of which areincorporated herein by reference.

BACKGROUND

This disclosure relates to a capacitor discharge circuit for dischargingcharges accumulated in a capacitor connected between alternating-currentpower lines.

Power supply circuits for electric devices of related art, a capacitorcalled an across-the-line-capacitor (hereinafter abbreviated as “Xcapacitor”), which suppresses unnecessary radiation, is provided betweenthe opposite ends of an alternating-current power source. A capacitordischarge circuit described in Japanese Patent No. 4446136 has beenknown as a conventional capacitor discharge circuit including an Xcapacitor.

This capacitor discharge circuit includes: an X capacitor connectedbetween alternating-current power lines; and a series circuit includinga first capacitor and a second capacitor that are connected between thealternating-current power lines and detect the voltage of analternating-current power source. To this series circuit are connected afirst time-constant circuit including a third capacitor, a firstresistor, and a second resistor, and a second time-constant circuitincluding a fourth capacitor, a third resistor, and a fourth resistor.The time constant of the first time-constant circuit is set to be lessthan the time constant of the second time-constant circuit. Accordingly,the charges of the third capacitor are discharged more quickly than thecharges of the fourth capacitor.

A transistor is connected to the outputs of the first time-constantcircuit and the second time-constant circuit. A series circuit includinga discharge resistor, a first switch, and a second switch is connectedbetween the opposite ends of the X capacitor. The collector of thetransistor is connected to the gates of the first switch and the secondswitch, and the emitter of the transistor is connected to the nodebetween the first switch and the second switch.

Here, when the plug is pulled out from a commercial power source, thecharges of the third capacitor are discharged through the first resistorand the second resistor and the transistor is turned off, but thecharges of the fourth capacitor remain. Then, by the charges remainingin the fourth capacitor, the voltage of fourth capacitor is applied tothe gates of the first switch and the second switch.

Thus, the first switch and the second switch are turned on, and thecharges of the X capacitor are discharged through the dischargeresistor. As a result, there is no potential difference between theinput terminals of the power supply circuit. Hence, the user can beprevented from getting an electric shock by touching the plug.

Meanwhile, alternating-current voltage detection is performed todischarge the charges remaining in the X capacitor when the commercialpower source is turned off. The alternating-current voltage detection isgenerally performed by dividing the voltage of the commercial powersource by using resistors.

SUMMARY

A capacitor discharge circuit according to one or more embodimentsincludes a capacitor connected in parallel to an alternating-currentpower source; a rectification element that performs full-waverectification on an alternating-current voltage from thealternating-current power source and supplies a rectified output to aload; a first discharge circuit that includes a first diode with ananode thereof connected to one end of the capacitor and a second diodewith an anode thereof connected to another end of the capacitor, andcauses the capacitor to discharge between cathodes of the first andsecond diodes and a ground terminal of the rectification element, thecathodes of the first and second diodes being connected to each other; afirst series circuit including a first capacitor and a second capacitorconnected in series between one end of the alternating-current powersource and the ground terminal of the rectification element; a seconddischarge circuit that causes the second capacitor to discharge suchthat an absolute value of voltage across opposite ends of the secondcapacitor, which is connected to the ground terminal side of therectification element, does not reach a predetermined voltage; and apredetermined period generator that actuates the first discharge circuitafter an elapse of a predetermined period of time from stoppage of adischarge operation of the second discharge circuit.

A capacitor discharge circuit according to one or more embodimentsincludes an X capacitor connected to an alternating-current power sourcein parallel; a rectification element that performs full-waverectification on an alternating-current voltage from thealternating-current power source and supplies a rectified output to aload; a first discharge circuit including: a first diode with an anodethereof connected to one end of the X capacitor; a second diode with ananode thereof connected to another end of the X capacitor, cathodes ofthe first and second diodes being connected to each other; a dischargeresistor; and a first switching element, the discharge resistor and thefirst switching element being connected in series between the cathodesof the first and second diodes and the other end of the X capacitor, afirst series circuit including a first capacitor and a second capacitorconnected in series between one end of the alternating-current powersource and a ground terminal of the rectification element; a seconddischarge circuit that includes a second switching element connected tothe second capacitor in parallel and a reference power source having apredetermined voltage, and turns on the second switching element when anabsolute value of voltage across opposite ends of the second capacitoris not equal to the predetermined voltage of the reference power source;and an activation circuit that has a timer and turns on the firstswitching element when a period of time during which the absolute valueof voltage across opposite ends of the second capacitor is equal to thepredetermined voltage of the reference power source is longer than apredetermined period.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the circuit configuration of acapacitor discharge circuit according to one or more embodiments;

FIG. 2 is a diagram illustrating paths of current during charge in thecapacitor discharge circuit;

FIG. 3 is a diagram illustrating paths of current during discharge inthe capacitor discharge circuit;

FIG. 4 is a timing chart illustrating an example of the waveforms ofsome elements in the capacitor discharge circuit;

FIG. 5 is a diagram illustrating the circuit configuration of acapacitor discharge circuit according to one or more embodiments;

FIG. 6 is a timing chart illustrating an example of the waveforms ofsome elements in the capacitor discharge circuit;

FIG. 7 is a diagram illustrating the circuit configuration of acapacitor discharge circuit according to one or more embodiments; and

FIG. 8 is a timing chart illustrating an example of the waveforms ofsome elements in the capacitor discharge circuit.

DETAILED DESCRIPTION

Capacitor discharge circuits according to embodiments of the inventionare described below in detail with reference to the drawings.

In the respective drawings referenced herein, the same constituents aredesignated by the same reference numerals and duplicate explanationconcerning the same constituents is basically omitted. All of thedrawings are provided to illustrate the respective examples only. Nodimensional proportions in the drawings shall impose a restriction onthe embodiments. For this reason, specific dimensions and the likeshould be interpreted with the following descriptions taken intoconsideration. In addition, the drawings include parts whose dimensionalrelationship and ratios are different from one drawing to another.

Embodiment 1

FIG. 1 is a diagram illustrating the circuit configuration of acapacitor discharge circuit according to one or more embodiments. InFIG. 1, the capacitor discharge circuit includes alternating-currentpower source AC, capacitors C1, C2, and C3, diodes D1, D2, and Da to Dd,switching elements Q1 and Q2, resistor R1, reference power sources V1and V2, comparators CP1 and CP2, OR circuit OR1, and timer 11. Switchingelements Q1 and Q2 each include an N-type MOSFET. Switching element Q1corresponds to a first switching element. Switching element Q2corresponds to a second switching element.

Capacitor C1, which is an X capacitor, is connected to the opposite endsof alternating-current power source AC, which generatesalternating-current voltage. Diode bridge circuit DB1 is connected tothe opposite ends of capacitor C1. Diode bridge circuit DB1 correspondsto a rectification element and includes four bridge-connected diodes Dato Dd.

The cathode of diode Dd and the anode of diode Da are connected to oneend of capacitor C1. The cathode of diode Dc and the anode of diode Dbare connected to the other end of capacitor C1. The anode of diode Dd isconnected to the anode of diode Dc, and the cathode of diode Da isconnected to the cathode of diode Db.

The anode of diode D1 (corresponding to a first diode) is connected tothe one end of capacitor C1, and the cathode of diode D1 is connected toone end of resistor R1. The anode of diode D2 (corresponding to a seconddiode) is connected to the other end of capacitor C1, and the cathode ofdiode D2 is connected to the one end of resistor R1 and the cathode ofdiode D1.

Diode D1, diode D2, resistor R1, and switching element Q1 correspond toa first discharge circuit. The first discharge circuit causes capacitorC1 to discharge between diodes D1 and D2 and the ground terminal ofdiode bridge circuit DB1 (the anode terminal of diode Dc).

The cathode of diode Da and the cathode of diode Db are connected to oneend of load 10, and the anode of diode Dd and the anode of diode Dc areconnected to the other end of load 10.

Capacitor C2 (corresponding to a first capacitor) and capacitor C3(corresponding to a second capacitor) are connected in series to one endof alternating-current power source AC and the ground terminal of diodebridge circuit DB1 (the anode terminal of diode Dc).

One end of capacitor C2 is connected to the other end of capacitor C1,and the other end of capacitor C2 is connected to one end of capacitorC3, the drain of switching element Q2, the non-inverting input terminal(+) of comparator CP1, and the inverting input terminal (−) ofcomparator CP2.

The other end of capacitor C3 is connected to the anode of diode Dd, theanode of diode Dc, the source of switching element Q2, the negativeelectrode of reference power source V1, the positive electrode ofreference power source V2, the source of switching element Q1, and theother end of load 10.

The non-inverting input terminal of comparator CP1 is connected to thenode between capacitor C2 and capacitor C3, and the inverting inputterminal is connected to the positive electrode of reference powersource V1. Reference power source V1 is a direct-current voltage of 0.1V, for example. The output terminal of comparator CP1 is connected to aninput terminal of OR circuit OR1.

Comparator CP2 has its inverting input terminal connected to the nodebetween capacitor C2 and capacitor C3, and has its non-inverting inputterminal connected to the negative terminal of reference power sourceV2. Reference power source V2 is a direct-current voltage of 0.1 V, forexample. The output terminal of comparator CP2 is connected to anotherinput terminal of OR circuit OR1. The output terminal of OR circuit OR1is connected to the gate of switching element Q2 and reset terminal R oftimer 11.

Comparator CP1, comparator CP2, reference power source V1, referencepower source V2, OR circuit OR1, and switching element Q2 correspond toa second discharge circuit. The second discharge circuit causescapacitor C3 to discharge such that the absolute value of the voltageacross the opposite ends of capacitor C3 does not reach or exceed apredetermined voltage, e.g. 0.1 V.

Timer 11 corresponds to a predetermined period generator. Timer 11 turnson switching element Q1, which is a part of the first discharge circuit,after the elapse of a predetermined period of time from stoppage of adischarge operation of capacitor C3. In other words, the predeterminedperiod generator is an activation circuit that turns on switchingelement Q1 by using a timer.

Specifically, upon input of an L level from OR circuit OR1, timer 11raises timer internal voltage. Then, when the timer internal voltagereaches a timer threshold, that is, when the predetermined period oftime elapses from stoppage of a discharge operation of capacitor C3,timer 11 determines that the input from alternating-current power sourceAC has been disconnected, and outputs an ON signal to switching elementQ1. Switching element Q1 is turned on by the ON signal from timer 11.

Note that the predetermined period of time is longer than a half cycleof alternating-current power source AC. More specifically, in FIG. 4,the period from time t4 to time t5 is equal to a half cycle ofalternating-current power source AC, and the period from time t6 to timet7, which is equal to the predetermined period of time, is longer than ahalf cycle of alternating-current power source AC.

Next, the operation of this capacitor discharge circuit 1 is describedin detail with reference to some drawings.

First, operation during charge of capacitors C2 and C3 is described withreference to FIG. 2. During charge of capacitors C2 and C3, asillustrated in FIG. 2, current flows through a route ofalternating-current power source AC→capacitor C2→capacitor C3→diodeDd→alternating-current power source AC to charge capacitors C2 and C3.Also, current flows through a route of alternating-current power sourceAC→diode Db→load 10→diode Dd→alternating-current power source AC.

Next, operation during discharge of capacitors C2 and C3 is describedwith reference to FIG. 3. In this case, current flows through a route ofalternating-current power source AC→diode Da→load 10→capacitors C3 andC2→alternating-current power source AC. Diode Dc is turned on when thedischarge of capacitors C3 and C2 is completed.

Next, the operation of some elements is described with reference to thetiming chart illustrated in FIG. 4. FIG. 4 illustrates P1 as the voltageof capacitor C2, P2 as the voltage of capacitor C3, CP1 as the outputvoltage of comparator CP1, CP2 as the output voltage of comparator CP2,the timer internal voltage, and Q1 as the ON-OFF output of switchingelement Q1.

First, as illustrated in FIG. 4, as the voltage of alternating-currentpower source AC rises, voltage P1 of capacitor C2 also rises in a halfsinusoidal waveform from time t1 to time t2. Specifically, voltage P1 ofcapacitor C2 changes at +dV/dt from time t1 to time t2.

As voltage P1 of capacitor C2 changes at +dV/dt, voltage P2 of capacitorC3 also rises according to +dV/dt and the ratio in capacity betweencapacitor C2 and capacitor C3. Then, when voltage P2 exceeds the voltageof reference power source V1, which is 0.1 V, comparator CP1 outputs anH level to OR circuit OR1. As a result, OR circuit OR1 applies an Hlevel to the gate of switching element Q2, so that switching element Q2is turned on. Hence, the charges of capacitor C3 are discharged.

As voltage P1 further rises, capacitor C3 is charged. When voltage P2exceeds the voltage of reference power source V1, which is 0.1 V,comparator CP1 outputs an H level to OR circuit OR1, so that switchingelement Q2 is turned on. As a result, the charges of capacitor C3 aredischarged. Thus, from time t1 to time t2, capacitor C3 is repetitivelycharged and discharged, as illustrated in FIG. 4. Also, the output ofcomparator CP1 is a pulse signal repetitively switching between an Hlevel and an L level. With the pulse signal through OR circuit OR1, thetimer internal voltage of timer 11 is also a pulse signal.

Then, as illustrated in FIG. 4, as the voltage of alternating-currentpower source AC drops, voltage P1 of capacitor C2 also drops in a halfsinusoidal waveform from time t3 to time t4. Specifically, voltage P1 ofcapacitor C2 changes at -dV/dt from time t3 to time t4.

As voltage P1 of capacitor C2 changes at -dV/dt, voltage P2 of capacitorC3 also drops according to −dV/dt and the ratio in capacity betweencapacitor C1 and capacitor C2. Then, when voltage P2 drops below thevoltage of reference power source V2, which is −0.1 V, comparator CP2outputs an H level to OR circuit OR1. As a result, OR circuit OR1applies an H level to the gate of switching element Q2, so thatswitching element Q2 is turned on. Hence, the charges of capacitor C3are discharged.

As voltage P1 further drops, capacitor C3 is discharged. When voltage P2drops below the voltage of reference power source V2, which is −0.1 V,comparator CP2 outputs an H level, so that switching element Q2 isturned on. As a result, the charges of capacitor C3 are discharged.Thus, from time t3 to time t4, capacitor C3 is repetitively charged anddischarged, as illustrated in FIG. 4. Also, the output of comparator CP2is a pulse signal repetitively switching between an H level and an Llevel. With the pulse signal through OR circuit OR1, the timer internalvoltage of timer 11 is also a pulse signal.

Then, from time t4 to time t5, when voltage P1 of capacitor C2 reaches0, comparator CP1 outputs an L level to OR circuit OR1. As a result,timer 11 raises the timer internal voltage from time t4 to time t5. Attime t5, voltage P1 rises as it does at time t1. Thus, the subsequentoperation is the same as the operation from time t1 to time t4.

Thereafter, when alternating-current power source AC is turned off, thatis, the plug is pulled out at time t6, voltage P1 of capacitor C2 ismaintained. In this period, voltage P2 of capacitor C3 is near 0.1 V,and OR circuit OR1 therefore outputs an L level to timer 11.

Thus, timer 11 raises the timer internal voltage from time t6 to timet7. At time t7, the timer internal voltage reaches the timer threshold,and timer 11 therefore outputs an ON signal to the gate of switchingelement Q1.

As a result, at time t7, switching element Q1 is turned on, so thatcurrent flows through a route of capacitor C1→diode D1→dischargeresistor R1→switching element Q1→diode Dc→capacitor C1. In this way, thecharges of capacitor C1 are discharged, thereby preventing the user fromgetting an electric shock by touching the plug.

In the capacitor discharge circuit described in Japanese Patent No.4446136, the series circuit with the first capacitor and the secondcapacitor, connected between the alternating-current power lines, have arelatively large capacity, and three or more configurations are neededfor the series circuit, time-constant circuits, and capacitors combinedwith each other. Hence, it is impossible to reduce the size of thecapacitor discharge circuit.

Also, in related techniques, the voltage of a commercial power sourcemay be divided using resistors to perform alternating-current voltagedetection. However, a resistive loss occurs, making it impossible toreduce the standby power. Also, in the case of performingalternating-current voltage detection using resistors, the resistorsneed to have high resistance, which requires ingenious ideas in terms ofprocessing, and increases the size of the actual mount area for theresistors.

In contrast, in the capacitor discharge circuit illustrated in FIG. 1,two voltage-detection capacitors C2 and C3 are used to detectalternating-current voltage, which makes the capacitor discharge circuitsimple. Thus, the size of the capacitor discharge circuit can bereduced. It is therefore easy to incorporate the capacitor dischargecircuit into an integrated circuit (IC).

Moreover, by using two voltage-detection capacitors C2 and C3 to detectalternating-current voltage, there is no loss due to detectionresistors. Thus, the standby power is further reduced. In addition,detection resistors having high resistance are not needed. Thus, thechip area of the capacitor discharge circuit can be reduced as well.

Note that discharge resistor R1 is used as a discharge device, but thecharges of capacitor C1 may be discharged through the activationcircuit.

Embodiment 2

FIG. 5 is a diagram illustrating the circuit configuration of acapacitor discharge circuit according to one or more embodiments. Thedifference of the capacitor discharge circuit illustrated in FIG. 5 fromthe capacitor discharge circuit illustrated in FIG. 1 is the followingconfiguration.

The capacitor discharge circuit illustrated in FIG. 5 includes resistorsR2 to R5 and reference power source Vref in place of reference powersource V1 and reference power source V2, illustrated in FIG. 1.

One end of resistor R4 (corresponding to a first resistor) is connectedto the other end of capacitor C3, and one end of resistor R5(corresponding to a second resistor) is connected to the other end ofresistor R4. The other end of resistor R5 is connected to the anode ofdiode Dd and the anode of diode Dc.

One end of resistor R3 (corresponding to a third resistor) is connectedto the source of switching element Q2, and the other end of resistor R3is connected to one end of resistor R2 (corresponding to a fourthresistor). The other end of resistor R2 is connected to the positiveelectrode of reference power source Vref (corresponding to a referencepower source), and the negative electrode of reference power source Vrefis connected to the anode of diode Dd and the anode of diode Dc.

The non-inverting input terminal of comparator CP1 and the invertinginput terminal of comparator CP2 are connected to the one end ofcapacitor C3 and the drain of switching element Q2. The inverting inputterminal of comparator CP1 is connected to the other end of resistor R3and the one end of resistor R2. The non-inverting input terminal ofcomparator CP2 is connected to the other end of resistor R4 and the oneend of resistor R5.

Note that the voltage at the node between resistor R4 and capacitor C3is P3, the voltage at the node between resistor R2 and resistor R3 isP4, and the voltage at the node between resistor R5 and resistor R4 isP5.

FIG. 6 is a timing chart illustrating an example of the waveforms ofsome elements in the capacitor discharge circuit. P1, CP1, CP2, thetimer internal voltage, and Q1 illustrated in FIG. 6 are the same asthose illustrated in FIG. 4. In FIG. 4, voltage P2 of capacitor C3changes in the range of the voltage of reference power source V2, whichis −0.1 V, to the voltage of reference power source V1, which is +0.1 V.

In the capacitor discharge circuit, comparators CP1 and CP2, switchingelement Q2, and OR circuit OR1, which serve as a second dischargecircuit, causes capacitor C3 to discharge such that the absolute valueof the voltage across the opposite ends of capacitor C3 does not reachor exceed voltage P4 at the node between resistor R2 and resistor R3, orcauses capacitor C3 to discharge such that the absolute value of thevoltage across the opposite ends of capacitor C3 does not drop to orbelow voltage P5 at the node between resistor R4 and resistor R5.

In this case, voltage P4 at the node between resistor R2 and resistor R3serves as the voltage of a reference power source for comparator CP1,and voltage P5 at the node between resistor R5 and resistor R4 serves asthe voltage of a reference power source for comparator CP2. Thus, theabsolute value of the voltage of capacitor C3 illustrated in FIG. 6changes in the range of voltage P5 of resistor R5 to voltage P4 ofresistor R2.

The other features of the operation of the capacitor discharge circuitaccording to embodiment 2 are similar to the operation of the capacitordischarge circuit according to embodiment 1, and their description istherefore omitted. Also, the capacitor discharge circuit according toembodiment 2 achieves similar advantageous effects to those by thecapacitor discharge circuit according to embodiment 1.

Embodiment 3

FIG. 7 is a diagram illustrating the circuit configuration of acapacitor discharge circuit according to one or more embodiments. Thedifference of the capacitor discharge circuit illustrated in FIG. 7 fromthe capacitor discharge circuit illustrated in FIG. 1 is the followingconfiguration.

The capacitor discharge circuit includes diode D3 in place of comparatorCP2, reference power source V2, and OR circuit OR1 illustrated in FIG.1.

The output of comparator CP1 is connected to reset terminal R of timer11. The cathode of diode D3 is connected to the drain of switchingelement Q2, and the anode is connected to the source. Reference powersource V1 is changed to a voltage higher than forward voltage VF ofdiode D3, e.g. to a voltage of 1 V.

FIG. 8 is a timing chart illustrating the waveforms of some elements inthe capacitor discharge circuit. P1, CP1, the timer internal voltage,and Q1 illustrated in FIG. 8 are similar to those illustrated in FIG. 4.In FIG. 4, voltage P2 of capacitor C3 changes in the range of thevoltage of reference power source V2, which is −0.1 V, to the voltage ofreference power source V1, which is +0.1 V.

In the capacitor discharge circuit, comparator CP1 and switching elementQ2, which serve as a second discharge circuit, cause capacitor C3 todischarge such that the positive value of the voltage across theopposite ends of capacitor C3 does not reach or exceed reference voltageV1. Also, the negative value of the voltage across the opposite ends ofcapacitor C3 is clamped at forward voltage V_(F) of diode D3.

Specifically, in the capacitor discharge circuit, capacitor C3 isdischarged by switching element Q2 only when voltage P1 of capacitor C2is at +dV/dt during charge (only when voltage P1 rises). Accordingly,the timing when the timer internal voltage starts to rise shifts to anearlier point; the timer internal voltage rises from time t2 to t5.Hence, the time taken for timer 11 to reach the timer threshold ischanged from the length of a half cycle of alternating-current powersource AC or longer to the length of a ¾ cycle or longer.

Note that, by changing the timer threshold, the capacitor dischargecircuit according to embodiment 3 can achieve similar advantageouseffects to those by the capacitor discharge circuit according toembodiment 1.

Application of Embodiments

In an application of any of the above embodiments, a switching powersupply device may be used as a load, and a control circuit for thisswitching power supply device may be configured as an integrated circuitwith the capacitor discharge circuit of the invention incorporated inthe control circuit.

Moreover, although above timer 11 is described as a time-constantcircuit, it may be configured as a counter circuit instead of atime-constant circuit.

The invention includes other embodiments in addition to theabove-described embodiments without departing from the spirit of theinvention. The embodiments are to be considered in all respects asillustrative, and not restrictive. The scope of the invention isindicated by the appended claims rather than by the foregoingdescription. Hence, all configurations including the meaning and rangewithin equivalent arrangements of the claims are intended to be embracedin the invention.

The invention claimed is:
 1. A capacitor discharge circuit comprising: acapacitor connected in parallel to an alternating-current power source;a rectification element that performs full-wave rectification on analternating-current voltage from the alternating-current power sourceand supplies a rectified output to a load; a first discharge circuitthat includes a first diode with an anode thereof connected to one endof the capacitor and a second diode with an anode thereof connected toanother end of the capacitor, and causes the capacitor to dischargebetween cathodes of the first and second diodes and a ground terminal ofthe rectification element, the cathodes of the first and second diodesbeing connected to each other; a first series circuit including a firstcapacitor and a second capacitor connected in series between one end ofthe alternating-current power source and the ground terminal of therectification element; a second discharge circuit that causes the secondcapacitor to discharge such that an absolute value of voltage acrossopposite ends of the second capacitor, which is connected to the groundterminal side of the rectification element, does not reach apredetermined voltage; and a predetermined period generator thatactuates the first discharge circuit after an elapse of a predeterminedperiod of time from stoppage of a discharge operation of the seconddischarge circuit.
 2. The capacitor discharge circuit according to claim1, wherein the predetermined period of time is longer than a half cycleof the alternating-current power source.
 3. The capacitor dischargecircuit according to claim 1, further comprising: a second seriescircuit including a first resistor and a second resistor connected inseries between the second capacitor, which is connected to the groundterminal side of the rectification element, and the ground terminal ofthe rectification element; and a third series circuit including a thirdresistor, a fourth resistor, and a reference power source connected inseries between the second capacitor and the ground terminal of therectification element, a negative electrode of the reference powersource being connected to the ground terminal of the rectificationelement, wherein the second discharge circuit causes the secondcapacitor to discharge such that the absolute value of voltage acrossthe opposite ends of the second capacitor is greater than a voltage at anode between the first resistor and the second resistor, or causes thesecond capacitor to discharge such that the absolute value of voltageacross the opposite ends of the second capacitor does not reach avoltage at a node between the third resistor and the fourth resistor. 4.The capacitor discharge circuit according to claim 1, wherein the firstdischarge circuit comprises: a discharge resistor connected to thecathodes of the first and second diodes; and a first switching elementthat is connected in series to the discharge resistor and is turned onby a signal from the predetermined period generator.
 5. The capacitordischarge circuit according to claim 1, wherein the second dischargecircuit causes the second capacitor to discharge only when voltage ofthe first capacitor rises, and the predetermined period of time islonger than a ¾ cycle of the alternating-current power source.
 6. Acapacitor discharge circuit comprising: an X capacitor connected inparallel to an alternating-current power source; a rectification elementthat performs full-wave rectification on an alternating-current voltagefrom the alternating-current power source and supplies a rectifiedoutput to a load; a first discharge circuit comprising: a first diodewith an anode thereof connected to one end of the X capacitor; a seconddiode with an anode thereof connected to another end of the X capacitor,cathodes of the first and second diodes being connected to each other; adischarge resistor; and a first switching element, the dischargeresistor and the first switching element being connected in seriesbetween the cathodes of the first and second diodes and the other end ofthe X capacitor, a first series circuit including a first capacitor anda second capacitor connected in series between one end of thealternating-current power source and a ground terminal of therectification element; a second discharge circuit that includes a secondswitching element connected in parallel to the second capacitor and areference power source having a predetermined voltage, and turns on thesecond switching element when an absolute value of voltage acrossopposite ends of the second capacitor is not equal to the predeterminedvoltage of the reference power source; and an activation circuit thatcomprises a timer and turns on the first switching element when a periodof time during which the absolute value of voltage across opposite endsof the second capacitor is equal to the predetermined voltage of thereference power source is longer than a predetermined period of time. 7.The capacitor discharge circuit according to claim 6, wherein thepredetermined period of time is longer than a half cycle of thealternating-current power source.